The palynologically defined Cretaceous-Tertiary boundary in the western interior of North America occurs at the top of an iridium-rich clay layer. The boundary is characterized by the abrupt disappearance of certain pollen species, immediately followed by a pronounced, geologically brief change in the ratio of fern spores to angiosperm pollen. The occurrence of these changes at two widely separated sites implies continentwide disruption of the terrestrial ecosystem, probably caused by a major catastrophic event at the end of the period.
An iridium abundance anomaly, with concentrations up to 5000 parts per trillion over a background level of 4 to 20 parts per trillion, has been located in sedimentary rocks laid down under freshwater swamp conditions in the Raton Basin of northeastern New Mexico. The anomaly occurs at the base of a coal bed, at the same stratigraphic position at which several well-known species of Cretaceous-age pollen became extinct.
ter all, the great success of the Talmi 6 approach shows the insensitivity of such transformations to configuration mixing.(3) Finally, the great sensitivity of the Ml rates to the small l± \ admixtures should remind us, once again, to exercise caution in the use of wave functions generated in a truncated space and serves to illustrate again that the important terms in the wave function are not always the largest but rather need to be determined anew for each specific operator. tWork performed in part under the auspices of the One of the significant aspects of spontaneous fission (SF) of the heavy nuclei is that the fragment mass distribution is strongly asymmetric for A ^ 254. From measurements of fragment kinetic energies for a series of nuclides ranging from 248 Cm up to 254 Cf and 254 Fm, Brandt et al. 1 note that the average mass of the heavy fragment remains relatively constant at A = 142 ± 1, while that of the light fragment increases. If this feature were to continue, a true symmetric distribution would not be reached until A approached 284. On the other hand, there has been some speculation that fission-fragment shell effects should accelerate the trend toward symmetry with the approach toward 264 Fm (=2 xl llSn Q2 ).Our measurement of the fragment kinetic energies from spontaneous fission of 257 Fm shows definite indications of the latter trend.The energy measurements were made with a pair of silicon surface-barrier detectors placed on opposite sides of a thin film on which the source was deposited. The amplified pulses were fed to a 512X512-channel two-parameter pulseheight analyzer; pulse heights of coincident events were stored on magnetic tape for subsequent computer processing.Two different sources of 257 Fm were employed.U.S.The first was isolated from debris from the "Hutch" nuclear explosion, 2 in which the mass-257 chain was produced by rapid multiple neutron capture in a uranium-thorium target. The second was produced by neutron capture in curium targets in the high-flux isotope reactor (HFIR) at Oak Ridge. Final purification of both preparations was accomplished by the standard ethanol-HC1 cation column elution for actinide-lanthanide separation, and by a hot alpha-hydroxyisobutyrate cation column elution for final isolation of fermium from other actinides. Purity of the sample was confirmed by alpha-pulse analysis. Solutions of the fermium were transferred to 35-to 40-Mg/cm 2 Zapon films and "freeze-dried" under vacuum to uniform spots 4 to 6 mm in diameter. Identical procedures were used to prepare 2B4 Cf sources (also from Hutch debris) and the 252 Cf standard sources used for energy calibration.The first (Hutch) source contained initially 0.8 SF/min and was measured over 32 weeks. A number of detectors and source distances were tried, and a total of 15 710 events were recorded. The second (HFIR) source started with 2.7 SF/ min and was measured over 75 days (17 951 events) in a constant source-detector geometry.The kinetic energy distribution of coincident fragments from spontaneous fissio...
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